What we know so far about the London Underground explosion

Detectives investigating an explosion on a London Underground train which sent passengers and those in the vicinity fleeing in fear have made two arrests.

Here is what we know about the incident so far:

:: An 18-year-old is in police custody having been detained on suspicion of being a terrorist by Kent Police in Dover, the arrest was described as "significant" by Deputy Assistant Commissioner Neil Basu.

:: A second man, aged 21, reported to be from Syria, was arrested in Hounslow on Saturday night, while his home in Stanwell, Surrey was searched by police on Sunday.

:: Both men are believed to have been fostered by Penelope and Ronald Jones, aged 71 and 88 respectively, who previously received MBEs for services to children and families.

:: Residents in the Cavendish Road area of Sunbury-on-Thames, Surrey, were told to leave their homes as armed police searched the couple's home.

:: Scotland Yard said it has spoken to more than 100 witnesses, while 180 images and videos have been sent to detectives by members of the public.

:: The national threat level has reverted back to severe, having been raised to critical in the wake of Friday's blast. It means a terror attack "highly likely".

:: Troops were dispatched to key sites across the country to free up armed police officers, with promises of "enhanced police presence" on the transport network.

:: Two victims remain in hospital, with 28 of the 30 injured having been discharged by Sunday, according to the latest update from NHS England. Police said most of those hurt were suffering from "flash burns".

:: Parsons Green station has reopened after the explosion was carried out towards the rear of a District Line train, bound for central London, at 8.20am on Friday.

:: Scotland Yard and the Prime Minister have rebuked US president Donald Trump over his claim the Parsons Green Tube bomber was "in the sights" of Scotland Yard.

:: The Islamic State (IS) terror group has claimed responsibility for the attack through its Amaq news agency, according to the US-based Site Intelligence.